Investigating the fact that the zoom lens with zoom rate between 1.3.about.2.0, in general, can be classified into two kinds as described in U.S. Pat. No. 4,516,839, the first kind of zoom lens is composed of the first group having Negative Refractive Power and the second group having positive Refractive Power, these kinds of zoom lenses have a serious shortcoming i.e. too long of its back focal length which makes the small type of lens very difficult the second kind of zoom lens is mainly an improved design in light of the shortcoming of the first kind of zoom lens. In order to shorten the focal length, it employs a telephoto type with the first group having Positive Refractive Power and the second group having Negative Refractive Power. The composition of this type of lens has shorter back focal length wherein the distance between the vertex of the first optical plane and the image plane is smaller than 1 comparing with the effective focal length.
But lens of projection type, because the size of the formed image of the television is far greater than the size of the negative, if one employs directly the mode of the two groups of the above mentioned two kinds of prior art, the zoom stroke is large, the size of the lens can't be diminished; besides, the size of the lens will increase.